The development of integrated circuits has resulted in a great increase in the number of circuits and circuit components per unit area in circuit layouts and has necessitated the development of new ways of interconnecting such circuits to the system in which they are to be used. One device developed to answer this new need has become ubiquitous and is commonly referred to as a DIP (dual-in-line) package or socket. An integrated circuit (I.C.) is mounted on an electrically conductive strip of metal which is formed to have two rows of leads. Thin wires are bonded between selected pads on the I.C. and respective leads. Typically this is then encased in plastic or ceramic to form an I.C. package with the leads depending downwardly therefrom in two parallel rows. A separate body or socket of electrically insulating material is formed with a pair of rows of recesses. A contact spring member is placed in each recess and is formed with a tail pin such as a solder tail or wire wrap pin extending through an aperture in the bottom wall of the recess. The body or socket is typically placed on a printed circuit board having selected apertures therein, along with other similar sockets, with the tail pins received in the apertures which are then fixed to the p.c. board. The I.C. package can then be removably mounted on a socket by pushing the downwardly depending leads into the recesses in the socket where they make electrical engagement with the spring contacts.
Over the years more and more circuit functions have been built into integrated circuit chips requiring an ever increasing number of pins for interconnection to the I.C. package. For practical considerations the maximum industry standard for DIP sockets has been set at 64 pins. For chips requiring even more inputs/outputs several approaches have been followed including the use of two double rows of pins. In addition a different lead layout has become common in which leads are directed to the extremity of four sides of the package to form a rectangular lead layout. The package is receivable in a mating socket having contact spring members arranged around a rectangular I.C. package seating surface with the spring members formed with tail pins similar to the DIP sockets but forming a rectangular layout. This type of socket is suitable for I/O's of up to 124 in number. Increased numbers of I/O's are not feasible because the distance of the lead from the I.C. to the pin becomes too long and adversely effects the time required for transmission of electrical signals. In addition, due to the closeness of the leads to one another, the lead frame typically must be made by etching rather than stamping which significantly adds to the cost of the device.
Functional circuit capacity and the physical size of printed circuit boards for large systems have continued to increase in response to performance demands. A recent improvement in interconnecting integrated circuits in such a system involves the use of a module comprising a layered substrate with a plurality of integrated circuit chips soldered to a surface thereof. Wiring interconnections are built into the substrate utilizing a number of copper power reference planes. A heat removal system is mounted in heat transfer relation with the I.C. chips and pins are brazed to metal pads on an opposite side of the substrate. An array of bifurcated springs each having a solder tail stem is placed at a selected site on a printed wiring board with the stems inserted in plated through holes and soldered to the board. A plastic frame fits over and aligns the springs. The module is aligned at the site, then it is moved toward the board such that the pins are adjacent the bifurcated springs and the module is then moved in a plane parallel to the surface of the board so that the pins are inserted into the bifurcations of the springs.
By way of example one such module is approximately 90 mm by 90 mm by 4.6 mm thick. One side has up to 118 I.C. chips soldered to its surface while 1800 pins are brazed to metal pads on the opposite side.
Due primarily to the extreme density of the input/output sites on the module substrate, the pin and bifurcated spring interconnection system is characterized by several limitations including problems involved with misalignment of one or more pins in an array. Although the bifurcated spring is flexible and can accommodate some pin misalignment, this capability is very limited since the spacing between pins in such an array is as little as 1.77 mm. Misalignment of pins will lead to costly rework or even scraping of the entire substrate and I.C. chips mounted thereon. Another limitation is the expense involved in brazing the pins to the metal pads on the substrate which is a relatively expensive manufacturing operation. Still another problem is a potential thermal expansion mismatch at varying temperature ranges between the substrate material (e.g. layered copper and alumina) and the pin material, typically an ion, nickel and cobalt alloy plated with a noble metal.